Haven't they already been selling the data?

My guess/suspicion is that in isolation the data's depth is pretty tame. The issue is when all the nitty gritty data is then tied with Facebook's.

IMO the bigger issue is that anyone integrating with them basically is telling Facebook about their app's usage stats now, which makes it easy for Facebook to see current and future competition as well as any emerging markets they can sink their fangs into. Adding Giphy is a great way to get your lunch eaten now as a startup in any remotely competitive space to Facebook.

To me it's kind of wild this type of acquisition doesn't violate some law (though not surprising) as this gives immediate historical data insight into dozens of direct competitors even if they stop integrating in the future.